25HP...VARIES WITH RPM
Yeah, sure. Well, looks like Merc outdid theirselves this time. If 2 cylinders were good, why not build a motor with 4? The early KF-9, KG-9 and Mark 40 were all very cool engines. These motors outran the competition hands down. Add a Quicksilver lower unit and voila, you've got yourself a Class D racer. This early four cylinder reigned supreme until 1956....then along came trouble.

Serial number ranges are:
Mk40H 712900 to 713399   and  825418 to 825917

MARK 30H & MARK 55H....1956
As time progressed, so did the technology.Merc had a few years of 4 cylinder practice under their belt and introduced two new race engines. A new 30 cubic inch racer, the Mark 30H, and the opened up and improved 40 c.i. Mark 55H. The 30H powerhead was the same as the fishing model except for a light flywheel. The 55H had it's own unique block and carbs. As with the 20 H, the 55H came equipped with Carter     N carbs.The same problems occured with this motor and most were eventually converted to Tillotson KA7A's. The early 30H came with sunset orange cowls. The early 55H was dark metallic green. They made later versions of both engines with the more square white cowls and chrome bands.
Serial number ranges are:
MK30H  997920 to 999919 and 1166471 to 1166568..with some others up to 1260724
MK55H  983920 to 984919 and  others 1260725 thru ?

       
pics of the original Carter Carb setup on the 55H

ALKY + NITRO = BOMB So , here's what happens when guys wanna blow their eardrums out.Guys like Quincy and a few other fellows really came out with a lot of innovations that the factory stole back and incorperated into the new engines on the market today....well...maybe not the pipes.